The mechanism of tumorigenesis by a bacterium, Agrobacterium tumefaciens, will be studied. Extracellular infections by this bacterium cause the formation of crown gall tumors. During the course of the infection, bacterial tumor-inducing (Ti) plasmid DNA is transferred from the bacterium to the host cell. These Ti plasmid DNA sequences are maintained and transcribed in the tumor cell. The interaction of the bacterium with tissue culture cells will be studied. Virulent strains of A. tumefaciens attach to the surface of host cells in vivo and in tissue culture. The ability of the bacteria to bind to tissue culture cells appears to be required for virulence. Bacterial mutants that fail to attach to tissue culture cells are avirulent. Revertants of these mutants selected for the ability to bind to tissue culture cells simultaneously recover virulence. The nature of the bacterial-binding site for attachment to host cells will be determined through comparisons of nonattaching mutant and attaching virulent parent bacteria. The host receptor that binds the bacteria will be characterized. It appears to consist at least in part of a surface protein. Changes in the bacteria subsequent to binding to host cells will be characterized. Avirulent mutants blocked at steps in tumor formation subsequent to attachment will be studied in order to elucidate the mechanism of transfer of plasmid DNA from the bacterium to the host cell. (R)